The field of art to which this invention is directed is hydrogel-forming polymer compositions made from crosslinked polyacrylic acid.
Water-insoluble hydrogel-forming polymers which are capable of absorbing large quantities of water and aqueous fluid are well known compositions. Such polymers, which are known as super-absorbent polymers, are lightly crosslinked acid functional polymers which swell in water or aqueous fluids but do not dissolve in the fluids. Super-absorbent polymers have been found to be particularly useful in diapers, feminine hygiene articles and surgical dressings. Descriptions of super-absorbent polymers and their uses are found in U.S. Pat. Nos. 3,669,103 and 3,670,731.
U.S. Pat. No. 4,654,039 (Reissue No. 32,649) is directed to hydrogel forming polymer compositions which are described as substantially water-insoluble, slightly cross-linked, partially neutralized polymers, derived from unsaturated polymerizable, acid group-containing monomers and cross-linking agents. Such polymers are made by polymerizing the acid monomer and the crosslinking monomer in water using a redox catalyst system, followed by partially neutralizing the acid groups with sodium hydroxide, then drying the polymer and pulverizing it to a powder.
British Patent No. 2,119,384 discloses super-absorbent polymers made by polymerizing in water acrylic acid in admixture with sodium acrylate and a crosslinking monomer using a persulfate catalyst, followed by drying and then heating with a crosslinking agent having at least two functional groups capable of reacting with carboxyl groups.
In U.S. Pat. No. 4,497,930, super-absorbent polymers are made by polymerizing acrylic acid in an inverse emulsion process followed by crosslinking the polymer with a diepoxide compound.
According to U.S. Pat. No. 4,295,987, super-absorbent polymers are made by polymerizing acrylic acid and a multifunctional acrylate monomer in water using a persulfate catalyst followed by neutralizing the acid groups with caustic and then blending in a divalent cation salt, e.g., zinc acetate, for additional crosslinking.
Numerous other patents disclose super-absorbent polymers and their uses, such as U.S. Pat. Nos. 4,076,663, 4,552,938, 4,507,438 and 4,535,098.
Many improvements have been made in the performance and properties of super-absorbent polymers over the years, such as in gel strength and reabsorbing capacity. However, such super-absorbent polymers do not have a balance of properties. Typically, polymers with high gel strength exhibit reduced reabsorbing capacity, which results, for example, in reduced diaper dryness. Polymers with increased reabsorbing capacity exhibit low absorbency under pressure and a reduced elasticity modulus which also reduces diaper dryness.
There is a need for a super-absorbent polymer having a balance of properties which, when the polymer is used in a diaper, results in improved diaper dryness.